Organic rankine cycle waste heat recovery system

ABSTRACT

A Rankine cycle for recovering waste heat from an engine includes a Rankine cycle circuit having a pump, a heat exchanger, an expansion device, a cooling device and a bypass circuit. The bypass circuit selectively provides a flow path for working fluid to bypass the expansion device, and includes a flow control valve actuated by a working fluid pressure differential to selectively open or close the bypass flow path. The expansion device includes a drive member directly coupled to a front end accessory drive (FEAD) of the engine such that a rotational speed of the expansion device is dictated by engine speed and the expansion device is free from separate speed control. The expansion device provides torque to the FEAD via the drive member when the system is in a waste heat recovery mode thereby reducing engine load and improving fuel economy.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Application Ser. No. 62/017,447 filed on Jun. 26, 2014. The disclosure of the above application is incorporated herein by reference in its entirety.

FIELD

This application relates generally to waste heat recovery systems and, more particularly, to an organic Rankine cycle waste heat recovery system, for example, for a motor vehicle powered by an internal combustion engine that includes a front engine accessory drive.

BACKGROUND

Up to 35% of the energy produced in a typical internal combustion engine is dissipated in the exhaust gasses. In such an engine, this exhaust heat is lost to the environment. Known Rankine cycles for diesel engines have limited exhaust temperature (up to only 750° C.), which necessarily serves to limit the amount of waste exhaust heat that can be recovered. Further, known Rankine cycles that convert waste exhaust heat to electricity have an added step of inefficiency caused by the alternator. Other conventional expanders associated with waste heat recovery systems use a clutching mechanism to decouple the expander from the accessory drive when there is not enough exhaust energy available for power recovery into the accessory belt. This clutch mechanism, however, drives additional cost and complexity into the expander and otherwise requires complex expander speed control. Thus, while conventional Rankine cycles for automotive applications work for their intended purpose, there remains a need for improvement in the relevant art.

SUMMARY

In accordance with one aspect of the invention, a Rankine cycle system for recovering waste heat from an engine of a vehicle is provided. In one exemplary implementation, the system includes a Rankine cycle circuit through which a working fluid circulates, and the circuit includes a pump, a heat exchanger, an expansion device, a cooling device and a bypass circuit. The pump circulates the working fluid in the circuit and the heat exchanger is thermally coupled to a heat source associated with the engine and is adapted to transfer heat to the working fluid. The expansion device is configured to selectively receive the working fluid from the heat exchanger and expand the working fluid to generate work. The cooling device is configured to cool the working fluid received from the expansion device. The bypass circuit selectively provides a flow path bypassing the expansion device, and includes a flow control valve actuated by a working fluid pressure differential to selectively open or close the flow path bypassing the expansion device. The expansion device includes a drive member directly coupled to a front end accessory drive of the engine such that a rotational speed of the expansion device is dictated by engine speed and the expansion device is free from separate speed control.

In one example implementation, the expansion device provides torque to the front end accessory drive via the drive member thereby reducing an overall load on the engine and improving fuel economy when the working fluid is directed to flow through the expansion device in a waste heat recovery mode of the Rankine system. In one example implementation, the flow control valve is actuated so as to provide the flow path bypassing the expansion device for the working fluid thereby minimizing a parasitic load of the expansion device while not controlling a rotational speed of the expansion device when the Rankine system is in non-waste heat recovery mode of operation.

In one example implementation, the expansion device is directly coupled to a belt wrap of the front end accessory drive and includes an absence of a clutch. In one example implementation, the expansion device comprises a scroll expander and the drive member comprises a pulley. The scroll expander includes a scroll shaft coupled to the pulley, which is directly coupled to the front end accessory drive.

In accordance with another aspect of the invention, a method of utilizing a Rankine cycle system for recovering waste heat from an engine of a vehicle is provided. In one example implementation, the Rankine cycle system includes a Rankine cycle circuit for circulating working fluid through a heat exchanger thermally coupled to a heat source from the engine, through an expansion device downstream of the heat exchanger, and through a condenser downstream of the expansion device. The method includes determining a pressure and a temperature of the working fluid in the circuit between the heat exchanger and the expansion device, and determining whether the system enters a waste heat recovery mode based on the determined temperature and pressure. A bypass valve in a bypass circuit of the Rankine cycle circuit that bypasses the expansion device is commanded to actuate a flow control valve to i) allow the working fluid to flow through the bypass circuit when the system does not enter the waste heat recovery mode thereby bypassing the expansion device; ii) block the working fluid from flowing through the bypass circuit thereby directing the working fluid to flow through the expansion device when the system is in the waste heat recovery mode; and iii) modulate a pressure of the working fluid at an inlet to the expansion device to optimize thermodynamic cycle efficiency. The expansion device includes a drive member that is directly coupled to a front end accessory drive of the engine such that a rotational speed of the expansion device is dictated by engine speed and the expansion device is free from separate speed control.

Further areas of applicability of the teachings of the present disclosure will become apparent from the detailed description, claims and the drawings provided hereinafter, wherein like reference numerals refer to like features throughout the several views of the drawings. It should be understood that the detailed description, including disclosed embodiments and drawings referenced therein, are merely exemplary in nature intended for purposes of illustration only and are not intended to limit the scope of the present disclosure, its application or uses. Thus, variations that do not depart from the gist of the present disclosure are intended to be within the scope of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an exemplary organic Rankine cycle waste heat recovery system for a motor vehicle in accordance with an aspect of the present disclosure;

FIG. 1A is a schematic illustration of an exemplary organic Rankine cycle waste heat recovery system for a motor vehicle in accordance with an aspect of the present disclosure;

FIG. 1B is a schematic illustration of an exemplary alternative bypass circuit for the organic Rankine cycle waste heat recovery system of FIG. 1A in accordance with an aspect of the present disclosure;

FIG. 1C is a schematic illustration of an exemplary alternative bypass circuit for the organic Rankine cycle waste heat recovery system of FIG. 1B in accordance with an aspect of the present disclosure;

FIG. 2 is a schematic of an exemplary vehicle exhaust system incorporating a boiler/vapor generator in accordance with an aspect of the present disclosure;

FIG. 3 is a schematic of an engine front accessory drive including an exemplary direct-driven clutchless expander in accordance with an aspect of the present disclosure;

FIG. 4 illustrates exemplary operation of a flow control valve of a bypass circuit at engine start and idle conditions in accordance with an aspect of the present disclosure;

FIG. 5 illustrates exemplary operation of the flow control valve of the bypass circuit during highway braking/coasting conditions in accordance with an aspect of the present disclosure;

FIGS. 6A-6B illustrate exemplary operation of the flow control valve of the bypass circuit in waste heat recovery modes in accordance with an aspect of the present disclosure;

FIG. 7 illustrates an exemplary flow diagram of operation of the organic Rankine cycle waste heat recovery system in a start up operational mode in accordance with the principles of the present disclosure; and

FIGS. 8A-8B illustrate an exemplary flow diagram of operation of the organic Rankine cycle waste heat recovery system in a steady state operational mode in accordance with the principles of the present disclosure.

DESCRIPTION

With initial reference to FIG. 1, an exemplary Rankine waste heat recovery system is schematically shown and generally identified at reference numeral 10. In accordance with various aspects of the application, the Rankine cycle waste heat recovery system is an organic Rankine cycle waste heat recovery system for an internal combustion engine and uses a clutchless expansion device or expander system with an expander bypass circuit for the working fluid. This allows an expander drive member or pulley of the expander system to free-wheel and be operated at a speed dictated by the engine, as will be discussed in greater detail below. In an exemplary implementation, the system recovers a portion of exhaust waste heat and converts it to shaft power to be fed back into the engine's accessory drive via the expander pulley. This reduces the overall load on the engine and therefore provides for improving highway fuel economy by an estimated 5-8%. In one exemplary implementation, the clutchless expander is a positive displacement clutchless expander. It will be recognized by one skilled in the art, however, that other forms of positive displacement expanders could be utilized, such as lobed expanders, piston expanders or Lysholm-type screw expanders.

An exemplary organic Rankine cycle waste heat recovery system in accordance with various aspects of the present application advantageously achieves vehicle fuel economy improvement by converting the waste exhaust heat directly into shaft power added to the accessory drive, thereby avoiding either the inefficiencies associated with driving an alternator and obviating the need for either reduction gearing or a selectable coupling/clutch when feeding shaft power directly into the engine's front accessory drive (FEAD). The present application therefore advantageously eliminates a need to incorporate complex expander speed controls.

By implementing an organic Rankine cycle, combustion waste heat is converted to rotational shaft power added to the FEAD, thereby reducing the overall load on the engine while otherwise minimizing expander parasitic load at times when there is insufficient exhaust energy available for power recovery. In accordance with another aspect of the application, the organic Rankine cycle exhaust waste heat recovery system advantageously accommodates exhaust temperatures up to about 900° C., thereby providing more exhaust heat for recovery and improving the thermal efficiency of the Rankine cycle system over known waste heat recovery systems. The disclosed system may further beneficially interface with the vehicle exhaust system and certain variations of the disclosed system interface with the EGR/intake system.

Continuing with reference to FIG. 1, the exemplary organic Rankine cycle waste heat recovery system (hereinafter “Rankine cycle”) 10 uses a R245fa working fluid, which is circulated through a Rankine cycle circuit or plumbing network 14, for example, by a mechanical or an electrically driven feed pump 18. In one exemplary implementation, the working fluid exits a receiver tank 17 and flows through the pump 18 as a liquid at relatively high pressure and low temperature. This liquid is then routed through a recuperator heat exchanger 22 that serves to increase the temperature of the working fluid such that the recuperator 22 output is a liquid at relatively high pressure and high temperature.

The working fluid is then routed through a boiler/superheater or vapor generator 26, which, in an exemplary implementation, is a stainless steel heat exchanger that transfers heat from the exhaust gas stream 30 (see also FIG. 2) into the working fluid. Along the length of the vapor generator 26 the working fluid temperature increases well beyond the vaporization temperature so that the output is a superheated vapor at very high pressure. As shown schematically in FIG. 2, an exhaust circuit 34 of an associated vehicle 38 is designed such that the boiler/superheater 26 can receive 0-100% of the exhaust flow or stream 30 from the engine 42, based on system needs. For example, a vapor generator control valve 52 and a vapor generator bypass valve 56 each receive control signals from an engine and/or vehicle controller or electronic control system 62 to control how much exhaust flow 30 is routed through the vapor generator 26 and how much is bypassed around the vapor generator 26 going directly out the tailpipe 68. In an exemplary implementation, the system 10 is optimized for maximum waste heat recovery by allowing as much exhaust flow to the vapor generator 26 as possible within the limits of the heat exchanger durability and the working fluid itself.

Returning to FIG. 1, the working fluid is then routed to an expansion device or expander 74, such as an exemplary scroll expander. In this exemplary implementation, the scroll expander 74 is a rotary device which expands the superheated vapor causing an energy release. The expansion energy is converted into shaft rotational power within the scroll. In an exemplary implementation and as best seen in FIG. 1A, attached to a scroll shaft 78 is a pulley 82, which is integrated into the belt wrap 88 of a belt 92 the front end accessory drive (FEAD) 98. As shown in the example in FIG. 3, the FEAD components included in the belt wrap 88 via associated pulleys can include, for example, an alternator 100, a tensioner 102, an idler 104, a coolant pump 106, a power steering pump 108 and a crankshaft 110.

The pulley 82 of expander 74 always rotates at a speed directly proportional to the crankshaft 110 and is not otherwise separately controlled in connection with the Rankine cycle waster heat recovery system 10. In the exemplary system 10, a clutchless, positive displacement expander 74 is driven by the engine 42 at low parasitic load when the system is not in waste heat recovery mode. However, it will also be recognized by one skilled in the art that parasitic loads could be reduced even further by adding a passive, sprag-type clutch mechanism between the expander 74 and the accessory drive 98.

When waste heat recovery is available, the expander 74 is driven by the Rankine cycle, transmitting power into the accessory drive 98, but still spinning at a speed that is directly proportional to engine speed. This recovered power reduces the overall load on the crankshaft 110 (main driving) pulley (FIG. 3). The reduced engine load results in less fuel consumed for the same amount of engine power to the wheels (improved fuel efficiency). The working fluid leaves the scroll expander 74 with less energy, as a reduced temperature, low pressure vapor.

Returning again to FIG. 1, after leaving the expander 74, the working fluid is routed through the recuperator 22. The recuperator 22 is a heat exchanger that transfers heat from the high temperature, low pressure vapor coming out of the scroll expander 74 to the high pressure, low temperature liquid coming out of the feed pump 18. This heat exchanger captures much of the remaining energy left in the working fluid after the scroll expansion and puts it into the working fluid upstream of the boiler/superheater 26 as a “pre-heating” device. The recuperator 22 improves the overall cycle efficiency by reducing both the amount of working fluid energy dissipated to the environment by the condenser and the required heat input to the system at the boiler/superheater 26. The recuperator 22 reduces the temperature of the working fluid to just above the saturation temperature so that the recuperator 22 output is a vapor at relatively low pressure and low temperature.

The working fluid is then routed, in the exemplary system, to an air or liquid cooled condenser 112, which transfers the remaining energy in the working fluid to the environment or to a coolant loop, respectively. In one exemplary aspect, the condenser 112 is a liquid cooled condenser associated with a discrete low temperature radiator circuit 116 having a low temperature radiator 120, as shown in FIG. 1. A phase change occurs in this heat exchanger (condenser 112) so that the working fluid leaves the condenser 112 as a low temperature, low pressure liquid. In the exemplary system, the heat added to the coolant of the low temperature radiator circuit 116 by the condenser 112 is passed to the low temperature radiator 120, where it ultimately leaves the system into the environment. Upon leaving the condenser 112, the working fluid returns to the feed pump 18, where the cycle starts over again. It will be recognized by one skilled in the art that other means for transferring energy out of the condenser 112 could be employed, such as a condenser cooled by air supplied by a fan, by dynamic pressure if in a moving vehicle, or a combination of both.

While the previously discussed vapor generator control valve 52 and vapor generator bypass valve 56 are the primary means of controlling the steady-state working fluid pressure at the inlet to the expander 74 in accordance with one exemplary aspect of the present disclosure, a control means with a low time constant is required to ensure functionality during transient conditions.

With continuing reference to FIGS. 1, 1A 2-3 and additional reference to FIGS. 4-6, exemplary operation of an expander bypass circuit 140 will now be discussed in greater detail. The expander bypass circuit 140 advantageously provides for, among other things, the ability to control the torque output of the expander at a speed dictated by the FEAD 98, thereby eliminating a need to specifically control the expander or utilize a clutch. FIGS. 4-6 illustrate one example of the bypass control circuit 140 having a pressure control valve system 144 in the exemplary form of a pressure control spool valve 148 and its attendant pilot control plumbing 152 that cooperate to advantageously control the torque output of the expander at the speed dictated by the FEAD.

Referring particularly to FIG. 4 and with continuing reference to FIGS. 1, 1A 2-3, at engine start (e.g., start up mode discussed herein) and idle conditions there is a low flow of low pressure vapor from the vapor generator 26 into the expander 74. After the expander 74, a portion of the superheated vapor is recirculated back to an inlet 158 of expander 74 via the expander bypass valve in the form of the pressure control spring-loaded spool valve 148. The amount of recirculated flow is dependent on a combination of the flow coming from the vapor generator 26 and the pumping requirements of the expander 74.

In one exemplary implementation, a spool 164 of the spool valve 148 is held in position by two opposing forces caused by two separate reference flow pressures; (i) a low reference pressure sensed at an outlet 168 of the expander 74 via a low pressure reference control line 172 of pilot control plumbing 152; and (ii) a high reference pressure sensed at the expander inlet 158 via a high pressure reference control line 178 of pilot control plumbing 152. Stated another way, the position of the spool 164 is determined by a pressure differential between the oppositely acting low and high working fluid reference pressures. As will be discussed in greater detail below, the low pressure from the low pressure reference control line 172 along with a valve spring 184 serves to urge or force the spool 164 into a bypass position (FIGS. 4 and 5). The high pressure from the high pressure reference line 178 serves to selectively urge the spool 164 to a blocking position (FIGS. 6A-6B) through control of a solenoid or electronically controlled valve 188 in the high pressure reference control line 178, as will also be discussed below in greater detail.

As can be seen in the exemplary implementation illustrated in FIGS. 1 and 4-6B, the low pressure reference control line 172 is in communication with the spool valve 148 and the plumbing network 14 just downstream of the expander 74 so as to sense pressure at the expander outlet 168. The high pressure reference control line 178 is in communication with the plumbing network 14 upstream of the expander 74 (or at the expander inlet 158) so as to sense pressure at the expander inlet 158. Line 178 is also in communication with a bypass flow line 194 and the spool valve 148 and spool 164 on an opposite side as the low pressure reference control line 172. Bypass flow line 194 is in communication with the plumbing network 14 downstream of the expander 74 and a controlled flow passage 198 of spool valve 148. As can be seen in FIGS. 4-6A, the high reference pressure control line 178 is in communication with an opposite end of spool valve 148 as the low pressure reference line 172 and selectively provides a force to the spool 164 to close or block the flow passage 198 through spool valve 148.

A fixed orifice 204 in the high pressure reference control or pilot line 178 ensures that a relatively small control solenoid 188 may be used, as the solenoid 188 manages reference pressures and not system flow. It will be appreciated that the control solenoid 188 may be advantageously operated in a pulse width modulation (PWM) mode so as to utilize the bypass spool valve 148 as an active pressure control element, if desired. As a result, the expander 74 free-wheels in this mode, simply recirculating the majority of the superheated vapor within an expander bypass loop 208 of the pilot control plumbing 152, while a small portion of the vapor out of the expander 74 continues on to the condenser 112.

With particular reference to FIG. 1B and continuing reference to FIGS. 1, 1A and 2-4, another exemplary implementation of the expander bypass circuit 140 schematically illustrated in FIG. 1A will now be discussed. In one exemplary implementation, a poppet 1164 of a relief valve 1148 is held in position by two opposing forces caused by two separate reference flow pressures: (i) a controlled reference pressure modulated between pressure sensed at an outlet 168 of the expander 74 via a low pressure reference control line 1172 of pilot control plumbing 1152, and pressure sensed upstream of the expander 74 (or at the expander inlet 158) via a high pressure reference control line 1177, and (ii) a high pressure present on the non-spring end of poppet 1164 via bypass valve port 1198. Stated another way, the position of the poppet 1164 is determined by a pressure differential between the oppositely acting low and high working fluid reference pressures, as will be discussed below in greater detail.

The low pressure reference control line 1172 is in communication with the relief valve 1148 and the plumbing network 14 just downstream of the expander 74 so as to sense pressure at the expander outlet 168. The high pressure reference control line 1177 is in communication with the plumbing network 14 upstream of the expander 74 (or at the expander inlet 158) so as to sense pressure at the expander inlet 158. The reference pressure control line 1178 is in communication with the high pressure reference control line 1177, the low pressure reference control line 1172, the relief valve 1148 and the poppet 1164. As can be seen in FIG. 1B, the reference pressure control line 1178 is in communication with an opposite end of relief valve 1148 as the bypass valve port 1198 and selectively provides a force to the poppet 1164 along with spring 1184 to close or block the flow passage 1198 and 1199 through relief valve 1148.

A fixed orifice 1204 in the high pressure reference control line 1177 ensures that a relatively small control solenoid 1188 may be used, as the solenoid 1188 modulates reference pressures with only a small proportion of system flow passing directly through it. It will be appreciated that the control solenoid 1188 may be advantageously operated in a pulse width modulation (PWM) mode so as to utilize the bypass relief valve 1148 as an active pressure control element, if desired. As seen in FIG. 1B, the reference pressure in control line 1178, plus the force from spring 1184, is holding the poppet 1164 in a closed position. The expander 74 free-wheels in this mode, simply recirculating the majority of the superheated vapor through a bypass check valve 2000, while a small portion of the vapor out of the expander 74 continues on to the condenser 112. FIG. 1C illustrates an alternate implementation where orifice 1204 is integrated into poppet 1164, thereby eliminating the complexities of the reference line network 1152.

Referring now to FIG. 5 and with continuing reference to FIGS. 1-4, at highway braking/coasting conditions for example, there can exist a transient of high flow, high pressure vapor into the expander 74, potentially in combination with low engine RPM (such as with a car equipped with an automatic transmission). Under this condition, the solenoid 188 can be fully opened or selectively modulated (pulse width modulation) to control temporary pressure overshoots on the high pressure side of the system 10. In this condition, the expander 74 is partially or completely bypassed and the superheated vapor flows directly on to the condenser 112 from either the bypass valve 148, or the expander 74, or a combination of both.

When there is sufficient exhaust energy for power recovery, the system goes into waste heat recovery mode, as shown for example in FIGS. 6A-6B. For example, the controller 62 can compare signals received from temperature and pressure sensors 278, 280, respectively, (FIG. 1) associated with the plumbing network 14 and/or system 10 to predetermined thresholds indicative of there being enough energy (temperature and flow) in the superheated vapor exiting the vapor generator 26. If the pressure and temperature of the superheated vapor are above the required predetermined thresholds, the system will enter into the waste heat recovery mode, where the controller 62 will command the solenoid 188 to close. This in turn will direct high pressure working fluid in the high pressure reference line 178 on the spool 164 of spool valve 148, which will overcome the combined valve spring 184 force and the force of the working fluid from the low pressure reference line 172 acting in the opposite direction.

As a result, the spool 164 will be urged toward the low pressure reference line 172 thereby closing or blocking the bypass flow passage 198. With the bypass passage 198 blocked and the solenoid valve 188 blocking flow of working fluid through the high pressure reference line 178 downstream of the spool valve 148, all or substantially all of the superheated vapor from the vapor generator 26 is directed to or forced to flow through the expander 74, with zero flow through the spool valve 148. In this condition, the expander pulley 82 is driven by the Rankine system 10 and torque is added into the accessory drive 98. During engine 42 transient conditions, the solenoid valve 188 can be selectively modulated (pulse width modulation) to control temporary pressure overshoots on the high pressure side of the system 10.

It will be appreciated that piston area ratios on the high and low reference pressure sides of the spool 164 and the inlet and outlet passage sizes 198, 199 can be varied relative to one another, and the spring rate of valve spring 184 can be varied, in order to match the action of the bypass valve 148 to the expander 74 size. It will also be appreciated that when the exemplary waste heat recovery system is used in combination with applications involving relatively steady-state loading of the host engine 42 and/or in combination with applications involving more feed forward control capability, such as a generator set, the pilot control solenoid 188 could be eliminated.

This clutchless expander system offers distinct advantages over an actively clutched expander system. It utilizes a simple mechanical bypass valve arrangement with a non-electrified spool mechanism, providing a relatively cost effective alternative to an actively clutched expander with its attendant control complexities. In addition, the parasitic load of the expander pulley 82 during non-waste heat recovery mode is minimized while avoiding the complexities of a clutched expander system. The addition of a relatively low-cost external solenoid provides the ability to mitigate transient pressure overshoots in the system and can be used as an active control element to control system high-side operating pressures.

In accordance with another exemplary aspect of the present disclosure, an attemperator 230 (FIG. 1) utilizing a PWM solenoid may be used to inject saturated liquid downstream of the vapor generator 26, or alternatively at an intermediate point in the vapor generator 26, to thereby control the superheat value at the expander 74. It will be appreciated that the injection of saturated liquid at an intermediate point in the vapor generator 26 achieves the further benefit of controlling the peak cycle temperature so as to avoid fluid over-temperature. The use of this exemplary attemperator 230 control provides a high gain control over the superheat temperature during transient conditions. The flow rate of the feed pump 18 is varied in order to provide normal, steady-state control of the superheat temperature at the inlet of the expander 74.

Turning now to FIGS. 7-8B, exemplary flow diagrams are shown in connection with system operational control of the organic Rankine cycle waste heat recovery system in a start-up mode (FIG. 7) and steady state mode (FIGS. 8A-8B). With initial reference to FIG. 7, the controller 62 determines if the vehicle 38 and/or engine 42 is powered on at block 250 and, if so, commands the feed pump 18 to pre-charge the plumbing network 14 and vapor generator 26 with the working fluid at block 254. This provides for, among other things, priming the vapor generator 26, which would not initially contain working fluid after the previous vehicle 38 shutdown due to, among other things, the manner in which the vapor generator 26 cools down and reaches equilibrium.

At block 258, the controller 62 determines if the engine 42 is running and, if so, continues to block 262, where the expander 74 bypass solenoid valve 188 is commanded to an open position, thereby placing the system 10 in an expander bypass mode of operation. At block 266, the controller 62 then commands the vapor generator bypass valve 56 to direct a controlled percentage of exhaust gas flow 30 into the vapor generator 26 or, stated differently, direct a controlled percentage of exhaust gas flow 30 to bypass the vapor generator 26. An engine 42 power request received by controller 62 can be the primary control for setting the exhaust bypass valve 56 position.

The controller sets the feed pump 18 flow to a predetermined percentage at block 270 such that there is a higher energy capacity of the working fluid in the plumbing network 14 of the system 10 than the exhaust energy into the vapor generator 26. The vapor generator 26 fluid exit temperature rate of rise is determined or calculated by the controller at block 274 utilizing a temperature signal from the temperature sensor 278 (FIG. 1) or the like. If the rate of rise is above a predetermined threshold, the controller 62 can modulate the feed pump 18 flow rate at block 284. If the rate of rise of the temperature is determined to be above the threshold by a predetermined amount, the optional attemperator 230 can be controlled to cool the working fluid in the manner discussed above.

At block 288, the controller 62 can optionally also determine the vapor generator 26 fluid exit pressure rate of rise utilizing a signal or information from the pressure sensor 280 at or near an exit of the vapor generator 26. The controller 62 can modulate the bypass solenoid valve 188 to bypass the expander 74 if the rate of rise of the fluid pressure is above a predetermined threshold at block 292. The controller 62 determines at block 296 whether the vapor generator fluid exit temperature is less than a predetermined threshold for entering a steady state mode of the system. If the temperature is greater than this predetermined threshold, then the system enters steady state mode of operation at block 302, which will be discussed below with particular reference to FIGS. 8A-8B. If, on the other hand, the exit temperature is less than this predetermined threshold, the controller 62 commands modulation of the solenoid valve 188 to increase the vapor generator 26 exit pressure at block 306 and commands the vapor generator bypass valve 56 to increase exhaust flow through the vapor generator 26 at block 308 and the process returns to before block 270.

In the steady state mode of operation and with reference to FIGS. 8A and 8B, the feed pump 18 flow is set to or maintained at the predetermined percent from the start-up mode at block 310. Similar to the start-up mode, the vapor generator 26 exit temperature rate of rise and exit pressure rate of rise are calculated or determined by the controller 62 at blocks 314 and 318, respectively. If the rate of rise of the temperature is greater than a predetermined threshold, the feed pump 18 flow rate can be modulated to lower the rate of rise at block 322. If the vapor generator 26 exit pressure rate of rise is greater than a predetermined threshold for steady state operation, the expander bypass solenoid 188 can be modulated to lower the pressure rate of rise at block 326. At block 330, the vapor generator 26 fluid exit temperature can be monitored via sensor 278 and the feed pump 18 flow rate can be modulated to lower the temperature if above a predetermined threshold at block 334.

At block 338, the vapor generator 26 fluid exit pressure can be compared to a predetermined threshold and, if less than this threshold, the controller 62 calculates or references the feed pump 18 upper flow limit at block 342. If the controller 62 determines the upper flow limit of pump 18 has been reached at block 346, the process continues to block 358. If the controller 62 determines at block 346 that the pump upper flow limit has not been reached, then the controller 62 commands modulation of bypass solenoid valve 188 to increase vapor generator fluid exit pressure at block 350 and commands the vapor generator bypass valve 56 to increase exhaust flow through vapor generator 26 at block 354. The process then continues to block 358. Returning to block 338, if the controller 62 determines that the vapor generator 26 fluid exit pressure is above the predetermined threshold, then the process continues from block 338 to block 358.

At block 358, the controller determines the condenser 112 exit pressure, such as with a pressure sensor 362, and determines whether this pressure is greater than a predetermined threshold. If the pressure is greater than a predetermined threshold, the controller 62 commands modulation of the expander bypass solenoid valve 188 to decrease vapor generator fluid exit pressure at block 366 and commands the vapor generator 26 bypass valve 56 to decrease exhaust flow 30 through the vapor generator 26 at block 370. If the controller 62 determines that the condenser 112 fluid exit pressure is less than the predetermined threshold at block 358, the process continues from there to block 374, where the controller 62 determines if the vapor generator 26 fluid exit pressure is now greater than a predetermined threshold. If yes, the controller 62 commands modulation of the bypass solenoid valve 188 to decrease vapor generator 26 fluid exit pressure at block 378 and commands the vapor generator bypass valve 56 decrease exhaust flow 30 through the vapor generator 26 at block 382. The process then continues to block 386.

Returning to block 374, if the controller 62 determines that the vapor generator 26 fluid exit pressure is below the predetermined threshold, then the process continues to block 386, where the controller 62 determines if the vapor generator 26 exit temperature is less than a predetermined threshold. If yes, the controller 62 commands modulation of the expander bypass solenoid 188 to decrease the vapor generator 26 exit pressure at block 390 and commands the vapor generator bypass valve 56 to decrease exhaust flow through the vapor generator 26 at block 394. From block 394, the process returns to before block 314 and repeats. Returning to block 386, if the controller 62 determines the vapor generator 26 fluid exit temperature is not less than the predetermined threshold, then the process returns to before block 314 and repeats.

Referencing the above discussion of exemplary operational control of system 10, the feed forward control system starts with lower amounts of exhaust energy entering the system in steady-state conditions and then lets the system utilize control functions to optimize equilibrium values for feed pump fluid flow and exhaust energy into the system. As can be seen from the above discussion, in the steady-state operating mode, the expander bypass solenoid valve 188 is primarily responsible for maintaining expander inlet pressure and its pressure regulation control characteristics in connection with the expander bypass circuit 140 are the primary vapor generator 26 exit pressure control. For cycle efficiency optimization in the steady state operating mode, the controller 62 monitors the expander 74 inlet pressure and, if it is low, the controller 62 commands the exhaust bypass valve 56 to add energy (increase exhaust flow) to the vapor generator 26 up to a point where the feed pump 18 reaches a predetermined limit based on the potential for exhaust waste heat recovery.

It will be understood that the mixing and matching of features, elements, methodologies and/or functions between various examples may be expressly contemplated herein so that one skilled in the art would appreciate from the present teachings that features, elements and/or functions of one example may be incorporated into another example as appropriate, unless described otherwise above. 

What is claimed is:
 1. A Rankine cycle system for recovering waste heat from an engine of a vehicle, the system comprising: a Rankine cycle circuit through which a working fluid circulates, the circuit including: a pump for circulating the working fluid in the circuit; a heat exchanger thermally coupled to a heat source associated with the engine and adapted to transfer heat to the working fluid; an expansion device configured to selectively receive the working fluid from the heat exchanger and expand the working fluid to generate work; a cooling device configured to receive the working fluid from the expansion device and cool the working fluid; and a bypass circuit selectively providing a flow path bypassing the expansion device, the bypass circuit including a flow control valve actuated by a working fluid pressure differential to selectively open or close the flow path bypassing the expansion device; wherein the expansion device includes a drive member directly coupled to a front end accessory drive of the engine such that a rotational speed of the expansion device is dictated by engine speed and the expansion device is free from separate speed control.
 2. The system of claim 1, wherein when the working fluid is directed to flow through the expansion device in a waste heat recovery mode of the Rankine system, the expansion device provides torque to the front end accessory drive via the drive member thereby reducing an overall load on the engine and improving fuel economy.
 3. The system of claim 2, wherein when the Rankine system is in a non-waste heat recovery mode of operation, the flow control valve is actuated so as to provide the flow path bypassing the expansion device for the working fluid thereby minimizing a parasitic load of the expansion device while not controlling a rotational speed of the expansion device.
 4. The system of claim 1, wherein the expansion device is directly coupled to a belt wrap of the front end accessory drive and includes an absence of a clutch.
 5. The system of claim 4, wherein the expansion device comprises a scroll expander and the drive member comprises a pulley, the scroll expander having a scroll shaft coupled to the pulley, which is directly coupled to the front end accessory drive.
 6. The system of claim 1, further comprising a discrete low temperature radiator circuit fluidly coupled to the condenser and configured to reject heat from the working fluid flowing through the condenser, the low temperature radiator circuit including a low temperature radiator.
 7. The system of claim 1, wherein the Rankine circuit further comprises a recuperator configured to reduce the temperature of the working fluid downstream of the expander.
 8. The system of claim 1, wherein the bypass circuit comprises: a low pressure reference control line fluidly coupled to the circuit proximate an outlet of the expansion device and to one side of the flow control valve; and a high pressure reference control line fluidly coupled to the circuit proximate an inlet of the expansion device and the flow control valve such that the low and high pressure reference control lines provide oppositely acting forces on the flow control valve.
 9. The system of claim 8, wherein the bypass circuit further comprises: a bypass flow line coupled at one end to the flow control valve and at another end to the circuit proximate to the expander outlet, the bypass flow line together with the flow control valve providing the flow path bypassing the expansion device; wherein the high pressure reference control line is coupled to the bypass flow line downstream of the coupling to the flow control valve such that flow of working fluid from the heat exchanger is in fluid communication with the flow control valve and the bypass flow line via the high pressure reference control line.
 10. The system of claim 9, wherein the bypass circuit further comprises an electronically controlled valve positioned in the high pressure reference control line between the coupling of the high pressure reference control line to the flow control valve and to the bypass flow line.
 11. The system of claim 10, wherein the electronically controlled valve is configured to be controlled to i) block flow of the working fluid through the high pressure reference control line to the bypass flow line thereby reducing bypass flow through the flow control valve; and ii) allowing the working fluid to flow through the electronically controlled valve to the bypass flow line and at least substantially bypassing the flow control valve.
 12. The system of claim 11, wherein the heat source comprises exhaust gas and the heat exchanger comprises a vapor generator; and wherein the system further comprises an exhaust gas bypass circuit having an exhaust bypass valve, the exhaust bypass circuit configured to selectively provide an exhaust flow path bypassing the heat exchanger.
 13. The system of claim 12, further comprising a controller in communication with at least the pump, the exhaust bypass valve, the electronically controlled valve and one or more temperature and pressure sensors in communication with the Rankine circuit proximate an exit of the heat exchanger.
 14. The system of claim 13, wherein the controller is configured to: receive signals from the one or more temperature and pressure sensors indicative of a temperature and pressure, respectively, of the working fluid exiting the vapor generator; and compare the signals to one or more predetermined thresholds indicative of whether the working fluid exiting the vapor generator comprises i) a sufficient or ii) an insufficient temperature and pressure for the Rankine system to enter a waste heat recovery mode.
 15. The system of claim 14, wherein the controller is further configured to command the electronically controlled valve to close when it is determined that there is a sufficient temperature and flow to enter the waste heat recovery mode, thereby actuating the flow control valve to block the working fluid bypass flow path and force the working fluid exiting the vapor generator to flow through the expansion device.
 16. The system of claim 14, wherein the working fluid flowing through the expansion device in the waste heat recovery mode is converted to shaft power and drives the pulley thereby providing torque to the front end accessory drive and reducing an overall load on the engine.
 17. The system claim 14, further comprising an attemperator in communication with the circuit proximate an exit of the vapor generator and configured to be controlled to selectively inject cooling fluid into the circuit to control a superheat value of the working fluid upstream of the expander.
 18. A method of utilizing a Rankine cycle system for recovering waste heat from an engine of a vehicle, the system including a Rankine cycle circuit for circulating working fluid through a heat exchanger thermally coupled to a heat source from the engine, through an expansion device downstream of the heat exchanger, and through a condenser downstream of the expansion device, the method comprising: determining, at a controller of the vehicle, a pressure and a temperature of the working fluid in the circuit between the heat exchanger and the expansion device; determining, at the controller, whether the system enters a waste heat recovery mode based on the determined temperature and pressure; and commanding a bypass valve in a bypass circuit of the Rankine cycle circuit that bypasses the expansion device to actuate a flow control valve to i) allow the working fluid to flow through the bypass circuit when the system does not enter the waste heat recovery mode thereby bypassing the expansion device; ii) block the working fluid from flowing through the bypass circuit thereby directing the working fluid to flow through the expansion device; and iii) modulate a pressure of the working fluid at an inlet to the expansion device to optimize thermodynamic cycle efficiency; wherein the expansion device includes a drive member directly coupled to a front end accessory drive of the engine such that a rotational speed of the expansion device is dictated by engine speed and the expansion device is free from separate speed control.
 19. The method of claim 18, wherein commanding the bypass valve to actuate the flow control valve to direct the working fluid to flow through the expansion device provides for the expansion device inputting torque to the front end accessory drive via the drive member thereby reducing an overall load on the engine and improving fuel economy; and wherein commanding the bypass valve to actuate the flow control valve to allow the working fluid to flow through the bypass circuit provides for minimizing a parasitic load of the expansion device while not controlling a rotational speed of the expansion device. 